Optical communication system

ABSTRACT

An amplifier arrangement includes an optical amplifier ( 10 ) with a source of injected power ( 11 ) for controlling amplifier gain. A feed forward gain control loop is provided with a weighting arrangement ( 50 ) capable of weighting the signal power at selected wavelengths input to the amplifier. In this way non-uniform spectral gain of the amplifier may be compensated for by selecting appropriate weighting factors. Thus the influence of wavelengths having a greater than average impact on the decay rate of the excited population within the amplifier is increased and vice versa. By tailoring the weighting arrangement to the specific class of amplifier utilised, and possibly to a specific amplifier within a class, amplifier gain can be precisely controlled and traffic signal power reliably held at a constant level regardless of link configuration changes.

FIELD OF INVENTION

[0001] The invention is broadly directed to optical transmission systemsutilising optical amplifiers. The invention has particular relevance tooutput power stabilisation in broadband optical amplifiers.

BACKGROUND ART

[0002] In optical communication systems, the limited dynamic range andsensitivity of optical receivers imposes certain requirements on thepower accuracy of traffic channels. Optical amplifiers, a term whichincludes Erbium doped fibre amplifiers, fluoride doped fibre amplifiers,Erbium Ytterbium amplifiers, Raman amplifiers, Brillouin amplifiers,semiconductor amplifiers, and the like, are now widely used in suchcommunication systems. A problem common to this class of amplifier isthe dependence of gain on the input power. In wavelength divisionmultiplexed (WDM) networks, this implies that for a fixed pump power,the amplifier gain is a function of the number of channels passingthrough the amplifier. Thus in reconfigurable networks, such as networksusing the dynamic addition or drop of optical channels, the gainexperienced by any one traffic channel will be affected by theconfiguration of the network as a whole. While slow power variations dueto ageing and improper fibre handling are normally mitigated byrelatively slow control feedback loops, transient suppression duringnetwork reconfiguration requires relatively fast control of theamplifier gain.

[0003] One method that is presently implemented to combat this problemis the use of a feed forward loop for active gain control. In such anarrangement, power input to the amplifier is measured and used tocontrol the power of the amplifier pump. If the input signal powerchanges, the pump power injected into the amplifier is adjusted to keepthe gain unchanged.

[0004] However, a characteristic of many optical amplifiers used inbroadband systems such as WDM communication systems is that the gainvaries depending on signal wavelength. Specifically, some inputwavelengths may experience different gains and have a correspondinglydifferent impact on the feed forward loop. This effect stems from thelevel of stimulated emission decay of excited states being higher whenthe input is at one wavelength than at another wavelength. The differentlevels of decay may also depend on the pumping levels. For example, inan Erbium doped fibre amplifier at high pumping levels, i.e. at a highlevel of population inversion, input wavelengths at around 1530 nm willprovoke a higher decay rate and thus a higher gain than those at around1550 nm at equal input powers. Thus for the same input powers a higherpump power must be applied in order to sustain this level of populationinversion when the input signal is at 1530 nm. For the same amplifier atlower pumping levels, wavelengths around 1550 nm may experience a highergain. Thus in this case for equal input powers a higher pump power wouldbe required to sustain a steady population inversion for an input signalat 1550 nm. For different amplifiers and different pumping levels, acurve of the decay of stimulated emission of the excited populationagainst wavelength may show several peaks centred around differentwavelengths. The traffic channels on the link will thus experiencevarying signal power.

[0005] It is thus an object of the invention to provide an opticalamplifier arrangement having stabile gain for signals over broadspectral range and that is substantially insensitive to rapid changes innetwork configuration.

SUMMARY OF INVENTION

[0006] According to the invention, there is provided an amplifierarrangement with a feed-forward gain-control loop with a weightingarrangement capable of weighting the signal power of selectedwavelengths received by the amplifier. In this way the non-uniformspectral gain of the amplifier may be compensated for by appropriateselection of the weighting factor. Hence the influence of wavelengthshaving a greater than average impact on the decay rate of the excitedpopulation can be increased and similarly the influence of thosewavelengths that have a smaller than average impact on decay rate can bereduced. By tailoring the weighting arrangement to the specific class ofamplifier utilised, and possibly to a specific amplifier within a class,amplifier gain can be precisely controlled and traffic signal powerreliably held at a constant level regardless of link configurationchanges.

[0007] The weighting may be performed in a single step using a filterwith a transfer function designed to vary with wavelength, such that theinput power in predetermined wavelength bands is attenuated or amplifiedas required.

[0008] In an alternative arrangement, the extracted signal power issplit into the required wavelength bands, and then subjected to apredetermined weighting function.

[0009] The invention further resides in a broadband opticalcommunications link including at least one of these amplifierarrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Further objects and advantages of the present invention willbecome apparent from the following description of the preferredembodiments that are given by way of example with reference to theaccompanying drawings. In the figures:

[0011]FIG. 1 schematically depicts an arrangement for stabilising gainof an optical amplifier in accordance with one embodiment of theinvention and;

[0012]FIG. 2 schematically depicts an arrangement for stabilising gainof an optical amplifier in accordance with a further embodiment of theinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 schematically depicts an amplifier arrangement connected ina wavelength division multiplexed (WDM) optical communications link. Theamplifier arrangement includes an active fibre 10 connected at an inputside to an upstream optical fibre 20 for receiving traffic signals andat an output side to a downstream optical fibre 30 for transmitting theamplified traffic signals.

[0014] The active fibre 10 is connected to a source of injected opticalpower 11, specifically a laser pump which pumps optical power into theactive fibre 10 and thereby determines the gain of the amplifier. Thepump power of this laser 11 is controlled by an injection current from again controller 110. Optical amplifiers that correspond to thisconfiguration include rare earth doped amplifiers, such as erbium-dopedamplifiers, fluoride doped amplifiers, Raman and Brillouin amplifiersand the like. An optical coupler or fibre tap 40 is connected to theupstream fibre 20 at the input side of the amplifier. This coupler 40extracts a small amount of the input power from the received trafficsignals. A filter 50 is connected to this coupler 40 and receives andfilters the extracted input signals. A photodetector 60 connected to thefilter 50 converts the optical signal output by the filter 50 into anelectrical signal, preferably a current The photodetector 60 may be aphotodiode, phototransistor, or any suitable opto-electric conversiondevice. The photodetector output is fed to the gain controller 110,which in turn adjusts the pump power 11, and thus the gain of theoptical amplifier in order to maintain the traffic signal powersubstantially constant regardless of the number of traffic channels. Theoptical coupler, 40, filter 50, photodetector 60 and gain controller 110together constitute a feed forward control loop for controlling the gainof the amplifier 10, 11 as a function of the input power.

[0015] This is necessary, since active fibre amplifiers, and also otherforms of optical amplifiers, such as semiconductor amplifiers and thelike, suffer from a common drawback, namely that the gain is dependenton the input power. If the number of traffic channels carried by themulti-channel optical link varies, the gain of the amplifiers will varyaccordingly. However, the gain curve of these amplifiers is rarely flatover the whole spectral range of input signals. The impact on the gainresulting from the addition or removal of a particular channel will thusdepend not only on the change in input power, but also on the wavelengthband of that channel. for example, at high pumping levels Erbium-dopedfibre amplifiers exert a higher gain on signals centred at around thewavelength of 1530 nm than signals centred at wavelengths of around 1550nm. Hence, if a channel centred around 1530 nm is added to an opticallink, the required pump power must be increased more than if a channelcentred around 1550 nm is added. This is assuming that the amplifier isdriven at a high overall gain.

[0016] The filter 50 compensates for the varying impact on amplifiergain by weighting or attenuating the different wavelengths in theextracted input signal differently. The filter 50 is essentially a bandpass filter with a transfer function H(λ) designed to pass thewavelengths of all channels that are or will be transmitted across theoptical link with varying degrees of attenuation. More specifically, thetransfer function H(λ) is designed such that the power of signals atselected wavelengths or wavelength bands are attenuated or amplified tocompensate for the opposite impact of these wavelengths on the gain ofthe optical amplifier to which the filter 50 is coupled. It will beappreciated that different classes of amplifier—and possibly individualamplifiers within the same class—will exhibit different gaincharacteristics across the spectral range of traffic signals. The filtertransfer function H(λ) is thus adapted to the specific characteristicsof the amplifier utilised. For example, in an amplifier driven at highgain, i.e. with a high level of population inversion, a filter 50connected to an Erbium doped fibre amplifier 10 will preferably have atransfer function that weights signals in the region of the wavelength1530 higher than signals of wavelengths in the region of 1550 nm. Inother words, signal wavelengths around 1550 are attenuated more thansignals of wavelengths of 1530 nm. But it should be noted that otherconfigurations are conceivable depending on how high the level ofpopulation inversion is. When this filtered or weighted signal is passedon to the photodetector 60, an electrical control signal results. Thiscontrol signal is used by the gain controller 110 to alter the gain ofthe amplifier 10 by adjusting the pump power such that the trafficsignal gain or gain per channel remains substantially constant.

[0017] In the preferred embodiment, the filter 50 is a dielectric filterdesigned using interference technology to provide a thin film filterthat includes a number of quarter wavelength thin films stacked on topof one another, with the wavelengths selected to provide the desiredwavelength attenuation pattern corresponding to the optical fibreamplifier utilised. It will be understood, however, that otherstructures may be used to form the filter. For example, the filter couldbe constituted by a series of fibre Bragg gratings, of single ofcascaded Mach-Zehnder interferometers, be of the Fabry-Perot type or beconstructed from a combination of the above techniques. It will beunderstood that the filter design is not limited to these examples, butthat other appropriate constructions known to the person skilled in theart may also be employed.

[0018] Turning now to FIG. 2 there is shown an alternative embodiment ofthe amplifier arrangement according to the invention. The basicarrangement shown in FIG. 2 is similar to that depicted in FIG. 1 andlike parts have been denoted by like reference numerals. Accordingly,FIG. 2 shows an optical fibre amplifier 10 coupled to a source ofinjected optical energy provided by a laser pump 11, which is controlledby a gain controller 110. The active fibre 10 is connected to an inputoptical fibre 20 and also to an output optical fibre 30. A coupler 40 isarranged on the input optical fibre 20 and extracts a small portion ofthe input signal power. This extracted signal is fed to a demultiplexer70, which is essentially a filter or series of filters arranged to splitthe signal power into selected wavelength bands designated by λ₁,λ₂-λ_(n). It will be understood that other components may be used inplace of the demultiplexor 70. These may include a diffraction grating,a photodiode array, which is described in K. Otsuka et al., ECOC'97,vol. 2, pp 147-150 (1997) or wavelength selective arrays. The opticalpower in each separate wavelength band λ_(n) is then converted toelectrical power using a photodetector 80, similar to the photodetector60 utilised in the arrangement of FIG. 1. The resulting electricalsignals are then each fed to a weighting module 90, which is preferablyan amplifier. Each weighting module 90 amplifies or attenuates thespectrally resolved signal power P₁, P₂-P_(n) by applying apredetermined amplitude weighting factor k₁, k₂ -k_(n) to the signal. Itwill be appreciated that the weighting modules may attenuate somewavelength bands, amplify other bands and leave some bands substantiallyunchanged. Hence the term ‘amplifier’ is used here to signify anamplifier arrangement that can apply a gain of less than unity as wellas above unity. The number n and width of the wavelength bands λ₁,λ₂-λ_(n) may correspond to those of the traffic channels transmittedover the optical link. Alternatively, the bands may be narrower or widerthan the traffic channels. Preferably the bands are selected such thatsignal wavelengths that have a higher than average impact on the gain ofthe optical amplifier 10 are isolated from the rest, so that they may beamplified or attenuated independently of the other signal wavelengths.

[0019] The weighting factors are determined at or prior to theinstallation of the amplifier arrangement in the optical link. They areselected to compensate for variations in the gain experienced bydifferent wavelengths passing through the fibre amplifier 10, 11 as aresult of the particular characteristics of the amplifier. Taking theexample of the Erbium doped fibre amplifier discussed above, this meansthat signals lying in a wavelength band centred around the wavelength of1530 nm will be weighted with a higher weighting factor than signals ina wavelength band centred around 1550 nm, such that the resulting signalpower of each weighted signal will have the same impact on the amplifiergain. The weighted signals are combined in an adder 100 and the totalweighted signal power used to control the gain of the amplifier 10 byaltering the injection current to the laser pump 11.

[0020] While in the arrangement shown in FIG. 2 the weighting functionis performed on electrical signals, this weighting may be performedinstead on optical signals, with the opto-electrical conversion using aphotodetector occurring prior to or even after combining the weightedsignals.

[0021] It will be appreciated that other classes of optical amplifiersthat are subject to gain variations over the spectral range of inputsignals can benefit from such a wavelength dependent feed forwardcontrol loop. The precise configuration of the amplifier arrangementwill naturally vary depending on the class of amplifier. Insemiconductor amplifiers, the amplifier gain is controlled by aninjection current. Hence in an amplifier arrangement incorporating asemiconductor amplifier, the injected power source denoted by referencenumeral 11 in FIGS. 1 and 2 would be a source of injection current orpump current.

1. An optical amplifier arrangement including an optical amplifier (10)a gain control arrangement (11), a feed forward control loop includingmeans (20) for measuring signal power input to said optical amplifier,and means (110) responsive to said measured signal power for adjustingthe gain of said amplifier controlled by said gain control arrangement,characterised in that said feed forward loop includes a weightingarrangement (50; 70, 90, 100) adapted to weight selectively the measuredsignal power as a function of wavelength prior to adjusting theamplifier gain.
 2. An arrangement as claimed in claim 1, characterisedin that said weighting arrangement includes a filter (50) with atransfer function (H(λ)) that varies with wavelength.
 3. An arrangementas claimed in claim 2, characterised in that said filter (50) is anoptical filter.
 4. An arrangement as claimed in claim 3, characterisedin that said optical filter (50) includes an interference filter, fibreBragg gratings and/or Mach-Zehnder filter.
 5. An arrangement as claimedin claim 1, characterised in that said weighting arrangement includesmeans (70) for splitting the signal power into at least two separatewavelength bands, means (90) for applying a weighting factor to thepower in each spectrally resolved band and means (100) for combining theweighted signal powers.
 6. An arrangement as claimed in claim 5,characterised by opto-electric conversion means (80) disposed betweensaid splitting means (70) and said weighting means (90).
 7. Anarrangement as claimed in claim 5 or 6, characterised in that saidsplitting means (70) includes a demultiplexer, diffraction grating orphotodiode array.
 8. An arrangement as claimed in any one of claims 5 to7, characterised in that said weighting means (90) include at least oneamplifier.
 9. An amplifier arrangement including an optical amplifier(10) with a source of injected power (11), a feed forward control loopincluding means (20) for measuring signal power input to said opticalamplifier, and means (110) responsive to said measured signal power foradjusting the gain of said amplifier by altering the power injected bysaid source (11), characterised in that said feed forward loop includesan optical filter (50) with a transfer function (H(λ)) that varies withwavelength for selectively weighting the measured signal power as afunction of wavelength prior to adjusting the amplifier gain.
 10. Anarrangement as claimed in claim 9, characterised in that said opticalfilter (50) includes an interference filter, fibre Bragg gratings and/orMach-Zehnder filter.
 11. An amplifier arrangement including an opticalamplifier (10) with a source of injected power (11), a feed forwardcontrol loop including means (20) for extracting signal power input tosaid optical amplifier, and means (110) responsive to said extractedsignal power for adjusting the gain of said amplifier by altering thepower injected by said source (11), characterised in that said feedforward control loop further includes a wavelength splitter (70) coupledto said power extracting means for splitting the measured input powerinto at least two signals in separate wavelength bands and means (90)arranged between said wavelength splitter (70) and said gain adjustingmeans (110) for applying a weighting factor to the power in eachspectrally resolved band.
 12. An arrangement as claimed in claim 11,characterised by opto-electric conversion means (80) disposed betweensaid wavelength splitter (70) and said weighting means (90).
 13. Anarrangement as claimed in claim 11 or 12, characterised in that saidwavelength splitter (70) includes a demultiplexer, diffraction gratingor photodiode array.
 14. An arrangement as claimed in any one of claims11 to 13, characterised in that said weighting means (90) include atleast one amplifier.
 15. A broadband optical communications link havingat least one amplifier arrangement, including an optical amplifier (10)a gain control arrangement (11), a feed forward control loop includingmeans (20) for measuring signal power input to said optical amplifier,and means (110) responsive to said measured signal power for adjustingthe gain of said amplifier controlled by said gain control arrangement,characterised in that said feed forward loop includes a weightingarrangement (50; 70, 90, 100) adapted to weight selectively the measuredsignal power as a function of wavelength prior to adjusting theamplifier gain.